The concentration of total, interstitial and perivascular collagen in the ventricular septum was studied morphometrically in patients with hypertrophic cardiomyopathy and in controlled subjects without heart disease. The quantity of total collagen was increased 4-fold and that of interstitial collagen 7-fold in patients with hypertrophic cardiomyopathy compared to normal controls. In patients with hypertrophic cardiomyopathy, no correlation existed between age, ventricular septal thickness, degree of myofiber disarray and quantity of collagen.